1. Field of the Invention
Gas streams, for example waste gas streams from chemical plants frequently contain vaporous components of substances foreign to air which lead to aerosol formation. Although as a rule the statutory limiting values for pollutants are met, the smoke plume associated with the aerosol formation is mostly troublesome. An intensified aerosol formation is frequently associated with specific malfunctions which can be precisely assigned and which it would be possible to assign even more precisely given precise knowledge of the aerosol quantity. The purely optically quantified aerosol quantity cannot always be determined absolutely with reference to the intensity of the smoke plume and is therefore problematical, since secondary effects such as, for example, the thinning of the aerosol in the ambient air, the vaporization of droplet aerosols, or the water vapor absorption of soluble, hygroscopic aerosols are superimposed. It is therefore difficult to undertake assignment of aerosol to operating states or malfunctions. The devising of countermeasures in the production area is therefore given over more or less to chance.
2. Description of the Related Art
Furthermore, aerosols in technical processes, in particular in plant combinations of large-scale chemical plants frequently lead to damage to catalysts, corrosion and product impurities. It has not so far been possible specifically to control the method processes with regard to avoiding aerosols or eliminating aerosols, since there is so far no safe, robust and reliable measuring technique. AT 395270 B discloses a method and an apparatus for absolute measurement of the numerical concentration of colloidal particles, in which an aerosol particle sample is supersaturated with vapor by adiabatic expansion in a measuring chamber, and this leads to uniform growth of the aerosol particles under consideration, their concentration subsequently being measured, by extinction of a coherent light beam penetrating the particle suspension concerned, simultaneously as a function of time with the light flux scattered at a predetermined angle.
A particular disadvantage of this method is that the aerosol particle sample to be investigated still has to be subjected before investigation to an adiabatic expansion for the purpose of supersaturation with vapor, and this is disadvantageous, in particular, for an online method.
It is known, furthermore, from U.S. Pat. No. 4,529,306 to monitor melted polymers for optical impurities by means of irradiation with preferably white light. In this case, a polymer, in particular a polymer melt is irradiated continuously by white light, and subsequently investigated for particles which refract white light by means of an optical detection unit. However, it is a disadvantage of this method that evaluation of the measurement results by image analysis can be applied starting only from particle sizes greater than 50 micrometers.
It was therefore the object of the present invention to avoid the indicated disadvantages of the prior art and, in particular, to make available a method and an apparatus which render possible an objective evaluation of particle properties, in particular in gases.
This object is achieved by an apparatus with the characterizing features of the present invention, and by a method with the characterizing features of the present invention.
Consequently, an apparatus according to the invention comprises a measuring chamber, preferably a flow-through cell, with entrance ports and exit ports for electromagnetic radiation, an emission source for electromagnetic radiation being provided and at least two detection apparatuses for determining the intensity of electromagnetic radiation scattered at the particles being provided, and the detection apparatuses detecting scattered electromagnetic radiation of different scattering regions.
It is therefore advantageously possible to apply the physical xe2x80x9cmultiangle scattering principlexe2x80x9d, in the case of which the sensitivity of the detection apparatus is adapted to specific particle size ranges by suitable selection of the detector positions which detect various scattering regions of the electromagnetic radiation.
Both gases which are at rest and those which are flowing are to be understood below under the term of gases for the purpose of their physical definition.
Physical collective parameters are to be understood below as, in particular, those properties and parameters of particles which are measured only as a mean value of the particle collective. Examples of this are concentration, mean particle diameter and the like.
It is understood by detection apparatus that this apparatus is suitable for detecting electromagnetic radiation and subsequently outputting an at least relative value of the electromagnetic radiation.
A first detection apparatus preferably detects scattered electromagnetic radiation from the forwardly scattered region (forward scattering). It is further preferred that a second detection apparatus detects scattered electromagnetic radiation from the backwardly scattered region (backscattering). It is thereby advantageously possible for aerosols with particle sizes of less than 10 xcexcm in general to be detected with particular precision by suitable selection of the detector position in the two previously mentioned regions. This also holds in the case of simultaneous occurrence of particles with particle sizes of more than 10 xcexcm in these gases.
In a particularly preferred embodiment, the forwardly scattered region comprises a forward scattering angle from 20 to 80xc2x0 and the backwardly scattered region comprises a backscattering angle from 100 to 160xc2x0, as a result of which the sensitivity to aerosols with a particle size of less than 10 xcexcm, in particular less than 5 xcexcm, can be detected with particular preference.
It is further preferred that means are provided for determining the ratio of the scattering intensities of the forward scattering and backscattering. This permits the mean particle size of the aerosol particles to be determined by simply bringing the two values into a relationship with one another. It is thereby preferably possible to detect mean particle sizes to 0.1 xcexcm. Particles with sizes above 10 xcexcm do not interfere in this case.
In a further preferred embodiment, means are provided for purging the emission source for electromagnetic radiation and/or for the detection apparatuses with at least one fluid. Purging with a fluid prevents a deposition of very fine particles on the optical system from leading to false measured values after a short operating time.
It is further preferred that means for multistage or continuous purging are provided. This ensures that even after a single purging very fine deposited particles still present are removed in a simple way from the optical system of the emission source of electromagnetic radiation and/or from the detection apparatuses, or that the deposits are prevented.
In a further preferred embodiment, it is provided that different fluids are provided for each purging stage. It is thereby preferably possible, for example, that a first purging is performed with air and very fine particles not removed and which would not longer be removable when dry are purged in a second purging with a liquid, for example water. Air is used again for purging in the third purging stage, with the result that after the liquid purging the optical apparatus is preferably blown dry by the purging line, thus excluding erroneous measurement owing to the deposition of drops of the purging liquid on the optical system.
In a further preferred embodiment, a third detection apparatus is provided which, for example, detects radiation with a mean scattering angle of 0xc2x0.
In addition to the simple detection, previously outlined, of aerosol particles of xe2x89xa610 xcexcm, it is thereby simultaneously possible to measure particles in the gas which have a size of more than 10 xcexcm, for example drops and dust. This measuring apparatus also permits precise differentiation between fine and coarse particles in a gas, and their simultaneous detection.
The emission source for electromagnetic radiation preferably emits wavelengths in the region of white visible light, ambiguity being avoided, in particular, in the determination of the particle size.
In a further preferred embodiment, means are provided for limiting the optically defined measurement volume, something which depresses the lower detection limits with reference to particle size and/or particle concentration owing to a higher radiation intensity in the measurement volume.
The above-named object of the present invention is achieved, furthermore, by a method according to the present invention.
It is provided thereby that the intensities of the scattered radiation of at least two different scattering regions are determined and their ratio is taken subsequently.
This advantageously permits simple determination of particle size, in particular in the range of xe2x89xa610 xcexcm.
In an advantageous embodiment, one scattering region comprises the forwardly scattered region, and the other scattering region comprises the backscattered region. It is further preferred that the forward scattering angle comprises 20 to 80xc2x0 and the backscattering angle comprises 100 to 160xc2x0, since thereby the sensitivity of the detection apparatus can advantageously be set to specific particle size ranges of less than 10 xcexcm.
In the method according to the invention, the intensity of the scattered radiation in a third scattering region is preferably measured, with particular preference for a mean scattering angle of 0xc2x0. The method according to the invention thereby makes available in a simple way the determination of particle sizes within the overall particle size range in gases, thereby rendering it possible simultaneously to measure and analyze in a simple way aerosols and also large particles in a gas.
The electromagnetic radiation preferably comprises wavelengths in the region of visible white light, since it is thereby possible in a simple way to avoid ambiguities in the determination of the particle size.
The method according to the invention is preferably used to determine physical properties of aerosols in gases which additionally contain particles which have particle sizes of more than 10 xcexcm.
Further advantages and refinements follow from the description and the attached drawings.
It goes without saying that the aforementioned features and those still to be explained below can be used not only in the combination respectively specified, but also in other combinations, or on their own, without leaving the scope of the present invention.